The term pig is used to refer to devices that are passed through pipelines or tubing whether for cleaning the pipelines or for monitoring the internal surfaces and thickness of the pipes or tubes. They may also be used for separation of product within the pipe or tube. This invention is particularly concerned with pigs that can be used to inspect pipelines or tubes from the inside to check for deposits and inconsistencies and irregularities in the walls of the pipelines or tubes. Inspection may be performed to assess the need for cleaning and/or pipeline repair or to assess the effectiveness of cleaning and/or repair. The invention further provides an integrated system providing combined activities whereby pipes or tubes can be sequentially cleaned with a cleaning pig and then inspected with a pig in which a common driving force is used to drive both types of pig through the pipelines or tube.
Although the invention is particularly useful with tubing used in oil refinery furnaces for carrying the hydrocarbons that are to be subject to high temperatures, it may also be used in connection with other pipes and tubing.
The tubing systems in refining furnaces such as crude oil distillation, vacuum thermal crackers, visbreakers, delayed cokers and the like typically have a sinusoidal path through the furnace to optimise the exposure of the contents of the tube to the heat; this is frequently referred to as the furnace coil being serpentine. In a typical furnace or process fired heater the product to be treated usually passes through a tube system which may be horizontal or maybe vertical in a downward and upward direction or a combination thereof and in some furnaces the initial section of the tubing consists of an upper closely packed tubing section in which the temperature of the product to be treated is raised to the treatment temperature by convection heating. Typically the pre-heated product then passes down to a lower section of the tubing in which there is more space between the lengths of tubing and in this section the tubes are heated by radiant heat. Typically, in both sections the tubes of a process fired heater consist of straight sections joined by bend sections, which may be semi-circular known as u-bends or may be box headers with (sharp bends) internal sharp 90 degree turns, sometimes referred to as “horseshoes” and/or “mule ears”.
In order for efficient and safe operation of such a tubular system it is important that the tubes are periodically cleaned and free from deposits and are also inspected to ensure the walls of the tube are free from undesirable deposits, tube material condition anomalies, wall thinning and/or various forms of metallurgic degradation. Inspections have been performed on line through furnace viewing windows and/or during furnace shut down through use of a variety of manual techniques used on the cleaned external surface of the tube walls. All these methods have limitations of usefulness as well as being time consuming and costly.
Additionally, where a furnace contains closely packed tubing such as is usually the feature of the higher level convection section, visual and manual inspection of tubes is impossible.
Accordingly it may be necessary to replace that section of the tubing according to the lifetime warranty provided by the supplier which can result in unnecessary replacement of tubing and also unnecessary and costly downtime of the furnace.
The inspection of the tube or piping may be performed sequentially after the cleaning of the internal surface of the tube or piping. Traditionally furnace process tubes have been cleaned/decoked using the method known as ‘steam air decoking’. More recently, since the mid 1990's, mechanical decoking or pig decoking has gained in favour in oil refineries around the world, widely replacing the practice of ‘steam air decoking’. Mechanical decoking is carried out by driving an abrasive or scraper pig through the pipe or tube to scrape deposits from the internal surface of the pipe or tube. This can be accomplished by driving the abrasive pig through the tube under fluid pressure such as water pressure. For example a cleaning unit having water tanks and pumps can be driven to a refinery, linked up with the tubing within a refinery furnace to produce a circuit through which the cleaning pig may be driven under water pressure so that the debris obtained by the cleaning operation is removed from the tubing system in the water stream and can be separated from the water for disposal. The cleaning operation may be performed by several runs of the cleaning pig which can be in the same direction or in opposite directions. Currently, after the cleaning operation the tubing system may be inspected in a separate operation.
As previously mentioned it is known to send a pig through a pipeline for the purpose of clearing any blockage therein and for removing unwanted deposits that have formed on the inner wall thereof. Such a device finds application, for example, in the oil industry, especially for cleaning fired heater or furnace tubes in a refinery. Refinery fired heaters may be subjected to temperatures normally in excess of 200° C. and in specialist furnaces temperatures can exceed 700° C. Such conditions lead to the formation of carbonaceous deposits (coke) on the pipeline wall. A pig can then be forced therethrough under pressure of a fluid, for example water, such that the deposits are removed by friction as the pig scrapes along the pipeline wall. U.S. Pat. No. 5,924,158 discloses an exemplary pig suitable for this purpose. The pig may be passed through the pipeline, uni-directionally or bi-directionally, several times to remove the coke. Decoking is carried out after the furnace has been taken out of service and cooled down.
However, the extreme conditions referred to above, usually exacerbated by the pressured flow of crude oil and semi-refined feedstock (oil) through the pipeline, can impose high levels of stress on the pipeline. Furthermore, high temperature refining activity within the furnace leads to separation of crude oil into its component parts, which can lead to corrosion of the tube wall. Decoking which is sometimes carried out by steam or air can also lead to a thinning of the tube wall. Additionally, there may be external corrosion of radiant furnace tubes by localised heat intensity from the furnace bumers which can lead to delamination of the tube metal.
Accordingly, regular monitoring of the condition of the tubing is required, to ensure that cleaning and/or decoking has been fully effective, and to ensure that the wall thickness has not been materially degraded by the cleaning or decoking or through the effects of the operation of the furnace in service. If a furnace tube is breached during operation in service this can be extremely dangerous potentially causing life threatening conditions. Similarly if a pipeline is allowed to deteriorate beyond safe limits, this can lead, in extreme cases, to a fracture, with the associated expensive and disruptive unscheduled downtime. Abrupt stoppages can also lead to blockage of the pipeline as the process materials carried thereby cool and increase in viscosity perhaps congealing.
Monitoring of the condition of tubing or a pipeline can be conventionally carried out by radiography, precision monitoring of flow and pressure, thermal imaging or hand held NDT (non-destructive testing). However, each of these techniques has disadvantages. Manual NDT can be time consuming, for example taking 6 or 7 days fully to inspect an entire furnace, and it also requires the furnace tube to be abrasively cleaned on the outer wall to carry out the inspection successfully. Furthermore, a furnace would normally need to be scaffolded internally to enable this work to be carried out, this being disadvantageous in time. Thermal imaging usually looks for hotspots as an indication of contamination although closely packed convection tubing cannot be inspected in this way. Monitoring is carried out whilst the furnace is in operation, and some areas of tube may not be visible from the access windows. Furthermore, the far side of tubes cannot be monitored by this technique.
It is also known to provide a tethered pig with monitoring equipment and to send it through a pipeline, in which operation of the equipment is controlled from outside the pipeline via an umbilical cable, a fibre-optic cable for example, and in which the responses detected by the on-board monitoring equipment are transmitted back along the cable to the external monitoring unit. However, such a monitoring pig is bulky, is not able to be used in pipelines of less than about 6 inches (15 cms) diameter, and is unable to navigate any useful distance through a serpentine tube coil such as will be found in a process fired heater.
European patent application publication EP 2039440 A1 describes a pipeline pig for, and a method of, monitoring a pipeline or tube coil in a convenient and an unmatched time efficient manner. The Pig is referred to as an “Intelligent Pig” abbreviated to “IP” and this will also be used in this application. EP 2039440 A1 further describes how the IP may be introduced into the tubular system following the cleaning operation by substituting the IP for the cleaning pig. This enables the same fluid driving and control system to be used for both the cleaning and inspection operations which is time saving and efficiency improving. This benefit is to some extent derived from the ability of the IP to perform its function without the need to contact clean metal on the internal tube wall. The IP produces an acoustic signal that travels through the fluid and echoes from the hard surface. The IP is able to distinguish between different materials and from interpretation of signals, it is possible to make judgments concerning different materials that may feature on tube wall surfaces.
Accordingly, the IP can be used during the course of the pig decoking and it is only necessary to establish a clear passageway so that the IP can securely pass from end to end.
This combination of pig decoking and pig inspection by the IP enables the IP to find areas where coke remains. This can helpfully and valuably impact on the decoking by guiding the decoking operators to areas where coke contamination remains, at the same time helping them to avoid wasting time running scraper pigs where no coke exists. This combination of decoking and inspection using the same machinery, equipment and manpower as combined activities provides considerable time efficiency.
The IP of EP 2039440 A1 is provided with flexible sleeves which help the IP to pass through a tube coil. This limits the danger of the IP becoming lodged in a tube coil having had its progress interrupted by patches of coke or other contamination.
EP 2039440 provides a pipeline IP for monitoring a pipeline from the interior thereof, the IP comprising an electronics module arrangement, which may be enclosed within an outer casing, wherein the electronics module arrangement comprises a transmitter for transmitting monitoring signals to the wall of the pipeline, a receiver arrangement for receiving transmitted signals returned from the wall of the pipeline, a microprocessor for analysing the received signals, a data logger for storing the data analysed by the microprocessor, and a source of electrical power for supplying the transmitter, receiving arrangement, microprocessor and data logger.
The IP of EP 2039440 is not dependent on a power supply that is external to the pipeline, and is able to analyse and store onboard the results of the monitoring for subsequent downloading when the IP has exited the pipeline. Furthermore the IP can be produced down to a size to allow it to pass through tubes with, for example, a diameter down to 85 mm. Since there is no tethering required for the IP in EP 203944 0, it can conveniently be sent through convoluted pipeline configurations from end-to-end, without the risk of snagging.
The IP of EP 2039440 comprises two sections joined by a universal joint. It contains a tapered nose section provided with sleeves which abut against the interior surface of the pipe to centre the IP in the pipeline and to protect the electronics. An electronics receiver unit is provided which is connected to a transmitter unit by means of the universal joint which helps the IP to pass around bend positions in the pipe. The rear section is provided with a petal sleeve that allows water to flow up against the sleeves at the nose section to drive the IP through the pipeline and it also centres the IP within the pipe.
In the system of EP 2039440 the pig is driven through the tubing system by fluid, preferably water pressure, against the rear section of the pig and the front end of the IP is provided with a tractor sleeve which is a flexible tapered sleeve that captures the driving fluid so that the IP is driven along the tube. The tractor sleeve fits tightly over the casing of the pig which is preferably of stainless steel. The sleeve is dimensioned so that it fits tightly up against the tube inner wall and is designed to provide a flexible ‘cup’ which seals firmly against the internal tube wall regardless of anomalous imperfections in the internal bore. The sealing effect of the tractor sleeve increases as the fluid pressure increases. The fluid fills the cup and forces the thinner flexible edges into the tube wall perfecting the seal and causing the IP to be controllable and smooth in its running. The fluid is preferably water. The IP is also provided with a rear ‘petal’ disc to allow water to flood around the acoustic carriage and fill the spaces between the central sections of the IP and the inner walls of the tube or piping and the petals of the disc should sit firmly up against the tube inner wall to ensure a) protection of the acoustic carriage from impact, and b) the centring of the carriage within the tube or pipeline.
Whilst the IP described in EP 2039440 has proved successful for the monitoring of pipes it has been found to suffer from certain disadvantages. Firstly its direction of movement within a pipe is unidirectional and it is not possible to reverse the direction of movement. Secondly there are some difficulties in using the IP of EP 2039440 in narrow bore pipes such as pipes of diameter of 76 mm or smaller and in particular the IP does not pass readily around the bridges at the ends of piping particularly in box headers, and some difficulties are experienced with narrow bore semi-circular sections. Thirdly the IP is heavier than is desired particularly as at times it is required to be driven vertically upwards in the pipes that are to be inspected.
The present invention addresses these disadvantages.